Purpose: Two of the earliest events in atherogenesis are the accumulation of low density lipoprotein (LDL) and macrophages in the arterial intima. Through in vitro and in vivo experiments this research tests two hypotheses: 1) arterial fluid dynamics and local oxidation of LDL activate the endothelium to express receptors for monocytes, and 2) localization of monocyte adhesion to vessel branch points represents a balance between hydrodynamic and adhesive interactions. Methods and Results: In our studies of biophysical aspects of monocyte interactions with endothelial cells activated with minimally modified LDL (mmLDL), we characterized adhesion molecule expression on human umbilical vein endothelial cells (HUVEC) after exposure to mmLDL for various lengths of time. Receptor expression was determined by flow cytometry. We find slight increases in ICAM after 4 h exposure to mmLDL and slight increases in E-selectin after exposure to mmLDL for 24 h. Flow chamber adhesion experiments were performed with U937 cells. A substantially increased arrest frequency was observed after 4 h exposure to mmLDL. The frequency of rolling cells was increased at higher shear rates after of endothelium to mmLDL for 4 and 24 h. Endothelium are exposed to a laminar shear stress of 10 dyne/cm2 for 4-24 and receptor expression measured by flow cytometry and image analysis. Studies are in progress to evaluate the effect of the flow field on adhesion molecule expression. An image analysis approach is being developed to relate image intensity to local adhesion molecule expression. The effect of adhesion molecule expression on monocyte adhesion is being investigated. We have performed experiments to examine monnocyte adhesion in a region of flow recirculation. Experiments are in progress to determine which receptors mediate adhesion. Monocyte adhesion in a region of flow recirculation in the presence of red cells was examined. The frequency of adhesive interactions and the duration of contact was increased around the reattachment point in the presence of red cells. Arrests were more common than rolling. The effect of red cells upon monocyte adhesion was much greater for monocytes adhering in recirculating flow than for monocytes adhering in shear flow. We have examined LDL permeability and accumulation by gamma counting of samples and autoradiography. Permeability was measured in experiments in which rabbits received 125I-LDL for 10 min. Permeabilities after one week of feeding 0.25% cholesterol are similar to those obtained in normocholesterolemic rabbits. After 3 and 4 weeks of feeding, the permeability appears to decrease, although significance was only demonstrated in the arch. When the tissue is subdivided between branch and nonbranch regions, preliminary results indicate that the permeability is greater around the branch than in nonbranch regions, although the permeability is not greater than that observed in normocholesterolemic rabbits as noted by others. Significance These studies are providing important insights into how fluid dynamics, LDL permeability and modification affect the localization of atherosclerosis. Future Plans Current and future experiments are to (1) characterize adhesion molecule expression after exposure of HUVEC to shear and recirculating flow for 24 h, (2) continue emasurements of adhession strength, and (3) continue measurements of LDL permeability in rabbits receiving antioxidants.